The rake classifier was developed at the turn of the century for minerals classification, but it has since been successfully employed in the fields of lime slaking and washing and dewatering sewage grit. These prior art machines consist of a rectangular tank having an inclined bottom, a rake structure within the rectangular tank positioned close to the inclined bottom surface of the tank, and a mechanism for operating the rake to conform to a predetermined reciprocating path.
The feed material or slurry to be treated is fed into the lower end of the tank. The liquid in the tank only partially fills it, with the deepest part of the liquid at the lower end of the tank. An outlet is provided through the end or side wall at the lower end of the tank to control the liquid level therein. In operation, then, the lower portion of the inclined rake is below the liquid level in the tank, while the upper portion of the rake is above the liquid in the tank.
The reciprocating movement of the rake which has an up-and-over motion rapidly separates the feed slurry into two fractions: the fine slowly settling material and the coarse, quickly settling material. The fines fraction is buoyant due to specific gravity differential and the agitation produced by the reciprocating action of the rake. Being unable to settle, the fines pass out with the liquid as overflow at the lower end of the tank. The coarse fraction, in contrast, sinks rapidly to the bottom of the tank. Settled grit advances up the inclined bottom surface of the tank with each upward or forward stroke of the rake. The up-and-over motion of the rake, in which the rake is alternately raised and lowered at the ends of its stroke, transports and then releases fines from the coarse material and keeps them in suspension until they overflow. Coarse particles are drained on a "beach" portion of the inclined bottom surface above the liquid level in the tank before being discharged from the tank at its upper end by the last blade of the reciprocating rake.
The mechanism in the prior art which produces the characteristic up-and-over movement of the rake is a system of heavy gears, pinions and cranks driven by an electric motor, V-belt, drive and heavy-duty gearing. As many as fifty to seventy individual parts are employed in this system. Early rake classifiers are shown in U.S. Pat. Nos. 849,379 to J. Van N. Dorr; 1,024,647 to D. J. Nevill; and 1,156,543 to D. J. Nevill.